Position Control Loop Research Articles

Motion Controller Design for Contour-Following Tasks Based on Real-Time Contour Error Estimation

Reduction of contour error is an important issue in contour-following applications. One of the common approaches to this problem is to design a controller based on contour error information. However, for the free-form contour-following tasks, there is a lack of effective algorithms for calculating contour errors in real time. To deal with this problem, this paper proposes a real-time contour error estimation algorithm. In addition, a motion control scheme that combines a position loop controller (which utilizes a velocity command feedforward) with a tangential-contouring controller (TCC) is employed to improve the contour-following accuracy. When implementing the TCC, a coordinate transformation matrix is needed. Unfortunately, it is difficult to calculate the coordinate transformation matrix when performing the free-form contour-following task. To overcome this difficulty and facilitate the use of contour controllers, a systematic approach is employed to derive the coordinate transformation matrix for the free-form contour-following tasks. Experimental results demonstrate the effectiveness of the proposed contour error estimation algorithm and the motion control scheme

The control performance improvement of the optical disk system by correction pulse control

AbstractThis paper proposes a control system which improves OPU (Optical Pick Up) control performance in an optical disk system. This system can be realized by adding a correction pulse control block which is based on a position error signal onto a conventional position control loop composed of the PID control rule. Using this function, the position error resulting from a physical distortion and acceleration disturbance to the disk can be decreased sharply. Furthermore, this system can decrease the deviation caused by the shock disturbance which cannot be controlled by the PID control rule. In addition, it becomes possible to decrease the overshoot in the impulse response sharply, and realize stable capture of a target position. Since this system can be created with a simple algorithm, it is possible to use it in a wide range of applications. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 160(2): 53–67, 2007; Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/eej.20570

Application of μ theory in compliant force control

Application of μ theory in compliant force control system is studied. Compliant force control strategy is developed based on the inner loop position control of 6-DOF parallel robot in order to simulate pushing and pulling procedure of forcible alignment of space docking. The cascade structure of "out-force-loop containing inner-position-loop" is depicted, and the method is set forth using classical control strategy to realize compliant force control. Considering uncertainties such as parameter perturbations, model perturbations and external disturbances, etc, robust force controller is designed using μ synthesis theory. It is listed in detail that the selection method of weighting functions in robust compliant force control loop and the designing procedure of robust force controller. Robust stability and robust performance are compared between robust force controller and classical force controllers by μ analysis. Experiment of robust force controller and classical force controllers is presented. The results indicate that the designed robust force controller is of efficiency and superiority.

Rigid Body Rate Inference from Attitude Variation

In this paper we research the extraction of the angular rate vector from attitude information without differentiation, in particular from quaternion measurements. We show that instead of using a Kalman filter of some kind, it is possible to obtain good rate estimates, suitable for spacecraft attitude control loop damping, using simple feedback loops, thereby eliminating the need for recurrent covariance computation performed when a Kalman filter is used. This considerably simplifies the computations required for rate estimation in gyro-less spacecraft. Some interesting qualities of the Kalman filter gain are explored, proven and utilized. We examine two kinds of feedback loops, one with varying gain that is proportional to the well known Q matrix, which is computed using the measured quaternion, and the other type of feedback loop is one with constant coefficients. The latter type includes two kinds; namely, a proportional feedback loop, and a proportional-integral feedback loop. The various schemes are examined through simulations and their performance is compared. It is shown that all schemes are adequate for extracting the angular velocity at an accuracy suitable for control loop damping.

Classical cascade and sliding mode control tracking performances for a xy feed table of a high-speed machine tool

The development of robust tracking controllers for linear feed drive, high speed machine tools is the primary objective of this work. Robustness implies the controller ability to preserve tracking accuracy in the presence of cutting forces and friction forces. This paper discusses the design and tracking performances of traditional cascade and sliding mode controllers for a xy feed table. Tracking performances are analysed through circular tests that are performed at selected tracking speeds and circle radius. For cascade P/PI controller, tracking error is proportional to the reference speed and is inversely proportional to the velocity gain value of the position loop controller. Speed and acceleration feed forward further reduce this tracking error. Quadrant glitches, a product of non-linear friction phenomena at the point of velocity reversal, are observed. Sliding mode controller, however, significantly reduces quadrant glitches and exhibits greater dynamic stiffness while its tracking accuracy is comparable to the cascade controller.

Current sensorless position–flux tracking controller for induction motor drives

An innovative indirect field-oriented output feedback controller for induction motor drives is presented. This solution is based on output feedback since only speed and position of the motor shaft are measured, while current sensors are avoided. This approach is suitable for low cost applications, where the position sensor cannot be removed to guarantee accurate position tracking. The proposed method provides global asymptotic tracking of smooth position and flux references in presence of unknown constant load torque. It is based on the natural passivity of the electromagnetic part of the machine and it guarantees asymptotic decoupling of the induction motor mechanical and electrical subsystems achieving at the same time asymptotic field orientation. Lyapunov analysis and nonlinear control design have been adopted to obtain good position tracking performances and effective torque–flux decoupling. The cascaded structure of the controller allows performing a constructive tuning procedure for speed and position control loops. Results of experimental tests are presented to demonstrate the tracking and robustness features of the proposed solution.

Application of μ Theory in Compliant Force Control

The application of μ theory in compliant force control system is studied. A compliant force control strategy is developed based on the inner loop position control of 6-DOF parallel robot in order to simulate the push and pull process of forcible alignment in space docking. Considering uncertainties such as parameter perturbations, model perturbations and external disturbances, etc., a robust force controller is designed using μ synthesis theory. The robust stability and robust performance are compared by μ analysis between the designed robust force controller and the classical force controller. The experiment results of the designed robust force controller and the classical force controller are shown. The results indicate that the designed robust force controller is of efficiency and superiority.

Simple and Robust Indirect Thrust Control for Positioning of Linear Induction Motors

Dealing with position control of Linear Induction Motors (LIM), most strategies in bibliography are based on Secondary Flux Oriented Control (SFOC) and Direct Thrust Control (DTC). However, SFOC of linear induction motors needs complex identification methods to compensate parameter variation during operation, mainly due to local heating and end-effects. On the other hand, DTC based methods for LIMs present thrust ripple and have problems at low and zero speeds.In this paper, a new Indirect Thrust Control (ITC) based strategy for position control of linear induction motors that makes up for these drawbacks is presented. The position control loop design methodology and the method for automatic adjustment of compensators are described. Experimental results are presented to evaluate the performance and sensitivity of the control strategy. Finally, some conclusions are drawn about the applicability of the new algorithm that demonstrate the main advantages versus SFOC and DTC.

An Observer-Based Friction Compensation Technique for Positioning Control of a Pneumatic Servo System

This paper presents a technique to cope with undesirable frictional effects in positioning control of a pneumatic actuator. This technique is based on an observer for estimating the dynamically varying friction force on-line. The observer is derived from an available nonlinear observer for Coulomb friction by modifying it to fit the pneumatic actuator and to enhance its estimation ability. Our pneumatic servo system for positioning consists of an inner pressure control loop, an outer position control loop, the friction observer, and a velocity observer. In this system, the friction force is compensated by adjusting a desired pressure value sent from the outer loop to the inner loop according to the friction observer output. Experimental comparisons with a conventional control system using friction compensation by means of accelerometer information feedback were carried out and show that our system works with almost the same high positioning accuracy as the conventional system, despite having neither an accelerometer nor a velocity sensor, and is more advantageous from the perspective of cost performance.

補正パルス制御による光ディスク装置の制御性能改善

This paper proposes a control system which improves OPU (Optical Pick Up) control performance in an optical disk system. This system can be realized by adding a correction pulse control block which is based on a position error signal onto a conventional position control loop composed of the PID control rule. Using this function, the position error resulting from a physical distortion and acceleration disturbance to the disk can be decreased sharply. Furthermore, this system can decrease the deviation caused by the shock disturbance which cannot be controlled by the PID control rule. In addition, it becomes possible to decrease the overshoot in the impulse response sharply, and realize stable capture of a target position. Since this system can be created with a simple algorithm, it is possible to use it in a wide range of applications. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 160(2): 53–67, 2007; Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/eej.20570

Simultaneous Measurement of Forces and Machine Tool Position for Diagnostic of Machining Tests

A diagnostic system of the milling process through the cutting forces and cutting tool position (coordinates X, Y, and Z) simultaneously collected is detailed in this paper. After the milling test, the cutting forces generated during the machining and the machine position where they occurred can be correlated. After collecting and recording of both the force and position signals, a graphic representation of force components can be applied. In this way, the machine user is able to check those events that happened during the milling; thus, a more useful diagnosis of machining problems is possible. The collecting system is composed by a dynamometric plate for the measurement of the three components of the cutting force, as well as an acquisition device connected to the analog output of the position control loops. After machining, the file containing position and force data is post-processed and chromatic and vector maps are generated. The analysis tool shows its capabilities being applied to three examples, referring to three research projects.

Recording of real cutting forces along the milling of complex parts

In this paper, a data acquisition system that simultaneously allows the recording of cutting forces and cutting tool position (coordinates X, Y and Z) is presented. Thus, the geometry of the surface being machined and the values of the cutting forces generated during the surface milling can be correlated. In this manner, two usual problems in milling experimentation are solved. First, those derive from the continuous changes in the feed rate value due to the special look ahead functions (being on-line applied by the numerical control in all high-speed milling centres), which affects the correlation between part geometry and forces. Second, those originate by the presence of unexpected stocks coming from previous semi-finishing operations, which changes the value of cutting forces respect to those calculated taking into account the theoretical tool engagement into the part. The objective of this work is the development of a diagnostics tool for allowing the detection of potential milling problems by researchers, making more profitable the performance of machining test on real complex parts. This tool can be used in the optimization of the milling of test-parts, machining real geometries that incorporate problems different from those observed in the linear tests at constant linear feeds (in end-milling conditions). The final goal of the system is the generation of cutting forces maps as a function of the geometry of real workpieces, which is an advance respect the usual forces recording in function of time or tool rotation typical of linear machining tests. In order to do that, the system incorporates a dynamometric plate for the measurement of the three components of the cutting force, as well as an acquisition card connected to the analog output of the position control loops. After machining, the file containing position and force data is post-processed and chromatic and vector maps are generated. The force analysis utility has been applied to three cases, in order to assess its feasibility in machining research projects. The first example shows a reduction of the number of tests and therefore machining time (even by 18 times) in the validation of a mechanistic cutting force model. Second example is focused on the detection of unexpected tool engagement conditions in complex parts. And the last one addresses to the milling of thin walls, for investigation of static and dynamic milling problems.

Comparison of Different Methods for Computing the Forward Kinematics of a Redundant Parallel Manipulator

In this paper, three numerical methods are presented to solve the forward kinematics of a three DOF actuator-redundant hydraulic parallel manipulator. It is known, that on the contrary to series manipulators, the forward kinematic map of parallel manipulators involves highly coupled nonlinear equations, whose closed-form solution derivation is a real challenge. This issue is of great importance noting that the forward kinematics solution is a key element in closed loop position control of parallel manipulators. The proposed methods, namely the Neural Network Estimation, the Quasi-closed Solution, and the Taylor series approximation, are using mainly numerical computations, with different ideas to solve the problem in hand. The latter two methods are proposed for the first time in literature to solve the forward kinematics of a parallel manipulator. These methods are compared in detail and the advantages or the disadvantages of each method in computing the forward kinematic map of the given mechanism is discussed. It is shown that a 4th order Taylor series approximation to the problem provides a good compromise for practical applications compared to that of other methods considered in this paper.

Robust and Precision Motion Control System of Linear-Motor Direct Drive for High-Speed<tex>$X$</tex>–<tex>$Y$</tex>Table Positioning Mechanism

In this paper, design and implementation of an H/sub /spl infin//-based precision motion control system is presented for a high-speed linear-motor direct-drive X-Y table positioning mechanism in semiconductor wire-bonding applications. The system works with a cascaded robust feedback control, which has an inner loop velocity controller and an outer loop position controller, and an autotuning feedforward compensator. The design aim is to achieve high and consistent tracking performance even in the presence of considerable resonance uncertainties and external disturbances. Toward this aim the velocity controller is designed using H/sub /spl infin// optimization technique, based on reduced-order modeling that considers three significant resonance modes and neglects all other resonance modes having an insignificant amplitude and/or too high frequency. These neglected modes and variations of the three resonance modes from machine to machine (due to manufacturing tolerance) and/or with different operating conditions are taken care of by appropriate additive uncertainty representation in the design phase. The resulting system is validated and implemented with a profile motion of a maximum acceleration of 5.2 g (1g=9.81 m/s/sup 2/) on mass-produced wire bonding machines.

Adaptive Inverse-Dynamic and Neuro-Inverse-Dynamic Active Vibration Control of a Single-Link Flexible Manipulator

This paper presents investigations into the development of adaptive inverse-dynamic and neuro-inverse-dynamic control strategies for a flexible manipulator system employing a combined collocated and non-collocated control structure. Collocated control is utilized to track the position of the system while the non-collocated inverse and neuro-inverse control are utilized to reduce the vibration of the system. The controllers are developed in two phases: a collocated position control loop using proportional-derivative feedback control is developed and combined first with an adaptive inverse non-collocated control loop using a recursive least-squares algorithm and then with a neuro-inverse model using a multi-layered perceptron neural network. The problem of instability of the non-collocated control loop arising from the non-minimum phase characteristics of the plant is solved in the former case by reflecting the non-invertible plant zeros into the stability region. In the case of the neuro-inverse model, the problem of instability of the control loop is accounted for through the neuro-inverse learning process. The performances of both the proposed control strategies are assessed within a simulation environment of a single-link flexible manipulator and it is demonstrated that a significant reduction in the level of structural vibration of the system is achieved with both techniques. The significance of the neuro-inverse model approach in achieving stable control is demonstrated.

A PWM Technique for DSP Controlled Ultrasonic Motor Drive System

In this study, a PWM technique is presented for digital signal processor controlled drive system of travelling-wave ultrasonic motor. The drive system is high-frequency two-phase serial-resonant inverter using mechanical resonant frequency of the ultrasonic motor. A TMS320F243 digital signal processor was used to control ultrasonic motor drive system. PWM signal was generated and then applied to the low pass filter to produce reference DC voltage level for speed control of ultrasonic motor. The driving frequency was selected as control input in the speed and position control loops. Experimental results show that the proposed drive and control system gives superior steady-state and transient performances in the speed and position control of ultrasonic motor.

IDENTIFICATION OF THE PNEUMATIC SERVO SYSTEM USING THE SELF-EXCITED OSCILLATIONS

The paper describes the identification of the variable critical gain and natural frequency of the pneumatic cylinder using the self-excited oscillations method. The pneumatic drives are characterized by the low stiffness of the pneumatic cylinder which depends on the piston position and determines the natural frequency of the cylinder. The variety of the stiffness in dependence on the piston position can be obtained using the curve describing the course of the critical gain and natural frequency in dependence on the piston position. This feature is a disadvantage by the closed loop control of the piston position. The low stiffness should be taken into the account by the design of the pneumatic servo system operated in the closed loop, because the natural frequency of the cylinder depends on the stiffness and limits the closed loop gain and quality of the step response of the controlled system. The identified curve can be used for the design of the adaptive controller with variable on the piston position dependent gain. The described approach is presented by the measurements and results realized on the test rig.

ROBUSTNESS ANALYSIS OF NONLINEAR SYSTEMS – APPLICATION TO INDUCTION MOTOR

This paper is dedicated to stability analysis of a position control loop based on an induction motor with flux oriented control in the general framework of μ-analysis. The pertinence of using this methodology is discussed in the general case of a smooth nonlinear system when the equilibrium point varies slowly. A new linear fractional representation of the model is presented with low repetition indexes of the parameters. Several algorithms are used for evaluating μ, bounds, allowing to compare them in term of accuracy and computation time.

Backstepping Position Control of a Hydraulically Actuated Clutch Servo System

Control of an electrohydraulic servo-actuated clutch for an automatic gear-shifting system for commercial cars is considered. A simplified model, which takes into account the fundamental behavior of the system, is mandatory for the design of the clutch control. The clutch position tracking controller, designed in the framework of backstepping technique, is based on a cascade structure with nested position, speed and pressure control loops. Feed-forward terms obtained through model inversion increase the dynamic performances, while closed-loop regulators provide robustness with respect to model uncertainties. In the controller design, nonlinearities of the mechanical actuator and of the servovalve are taken into account in order to improve achievable performances. The controller behavior during tracking of smooth position references has been experimentally tested.

An Automatic Depth Control System for Online Measurement of Spatial Variation in Soil Compaction, Part 3: Design of Depth Control System

To measure the soil compaction online by measuring the draught on a cutting tool or subsoiler, the moisture content should be known and the cutting depth should remain constant. In this paper the depth controller of a subsoiler is designed. The three-point hitch is used as a correction mechanism to adjust the cutting depth by utilising the signal output of a depth sensor, which consisted of a swinging arm metal wheel and a linear variable displacement transducer (LVDT). An extra control loop is constructed around the existing position control loop of the three-point hitch. Based on a semi-analytical model multiplied by a conversion factor, a proportional (P) controller, a lead compensator, a proportional integrative derivative (PID) controller and a PID controller with filtered derivative (D), actions are designed. All controllers provided satisfactory results, with the latter showing the best performance in practice. It could also be demonstrated that, with the hardware used, no further improvement can be achieved by increasing the complexity of the controller.